Add a ZonedTime type

[sffc]

It can live next to ZonedDateTime.

Why: in order to use TimeFormatter to format a time and a zone together.

Other solutions considered but infeasible:

1. Use ZonedDateTime: no, it has a calendar, but TimeFormatter has the () calendar
2. Implement InFixedCalendar<()> for ZonedDateTime: no, this breaks type inference for everyone else
3. Add format_any_calendar to TimeFormatter: possible, but seems confusing; ZonedTime is better

@Manishearth @robertbastian